README.md

How to use std::map for reverse sorting

std::map provides a key to value mapping, where the key is sorted. Behind the scenes std::map could be implemented as a tree or hashtable, but the point is you don't need to worry about that. All you need to do is provide a means to sort the keys via the "<" operator.

However, for standard provided classes you don't even need to do that e.g.:

    std::map< std::string, int> m;

But what is the sort order? Well it is alphabetically increasing, but you can be specific and say:

    std::map< std::string, int, std::less<std::string> > m;

Or alternatively for reverse sorting:

    std::map< std::string, int, std::greater<std::string> > m;

    m["zaphod"] = 1;
    m["universe"] = 2;
    m["arthur"] = 3;
    m["marvin"] = 4;
    m["mice"] = 5;
    m["vogon"] = 6;

yields

    arthur 3
    marvin 4
    mice 5
    universe 2
    vogon 6
    zaphod 1

Here is the full example:

#include <algorithm>
#include <iostream>
#include <list>
#include <map>
#include <sstream>
#include <string>

static void backward_sort(void)
{
  // backward sorted map
  std::map< std::string, int, std::greater< std::string > > m;

  m[ "zaphod" ]   = 1;
  m[ "universe" ] = 2;
  m[ "arthur" ]   = 3;
  m[ "marvin" ]   = 4;
  m[ "mice" ]     = 5;
  m[ "vogon" ]    = 6;

  for (auto i : m) {
    std::cout << i.first << " " << i.second << std::endl;
  }
}

static void forward_sort(void)
{
  // forward sorted map
  std::map< std::string, int, std::less< std::string > > m;

  m[ "zaphod" ]   = 1;
  m[ "universe" ] = 2;
  m[ "arthur" ]   = 3;
  m[ "marvin" ]   = 4;
  m[ "mice" ]     = 5;
  m[ "vogon" ]    = 6;

  for (auto i : m) {
    std::cout << i.first << " " << i.second << std::endl;
  }
}

static void default_sort(void)
{
  // default sorted map
  std::map< std::string, int > m;

  m[ "zaphod" ]   = 1;
  m[ "universe" ] = 2;
  m[ "arthur" ]   = 3;
  m[ "marvin" ]   = 4;
  m[ "mice" ]     = 5;
  m[ "vogon" ]    = 6;

  for (auto i : m) {
    std::cout << i.first << " " << i.second << std::endl;
  }
}

int main(int, char **)
{
  backward_sort();
  forward_sort();
  default_sort();
}

To build:

cd std_map_reverse_sort
rm -f *.o example
clang -std=c++2a -Werror -g -O3 -fstack-protector-all -ggdb3 -Wall -c -o main.o main.cpp
clang  main.o -lstdc++  -o example
./example

Expected output:

# backward sorted map
zaphod 1
vogon 6
universe 2
mice 5
marvin 4
arthur 3

# forward sorted map
arthur 3
marvin 4
mice 5
universe 2
vogon 6
zaphod 1

# default sorted map
arthur 3
marvin 4
mice 5
universe 2
vogon 6
zaphod 1